Fix #413: paste no longer adds '_' label to conductors that had no label

[ispyisail]

Problem

When erase-label-on-copy is enabled (the default), copy-pasting conductors that had no label (num="") caused them to gain a _ label on the paste. The user's original conductor was visually clean (no text), but every copy came out labelled, requiring manual cleanup.

Root cause (traced through the code)

PasteDiagramCommand::redo() in diagramcommands.cpp reset every pasted conductor to the diagram's default text:

cp.text = c->diagram() ? c->diagram()->defaultConductorProperties.text : "_";

ConductorProperties initialises text to "_" by default (conductorproperties.cpp:246), so diagram->defaultConductorProperties.text is "_" unless the user has explicitly changed it in Project/Diagram Properties. A conductor that arrived from the clipboard with text = "" (from num="") was unconditionally overwritten with "_".

Additionally the conductor-reset loop was nested inside the per-element loop, so with N pasted elements the conductors were reset N times (harmless but wasteful).

Fix

Two changes in diagramcommands.cpp:

1. Move the conductor-reset loop outside the per-element loop — runs once per paste, not once per element.
2. Guard the reset: only reset conductors that already carried a non-empty label. Conductors with text == "" keep their empty label after paste.

if (!cp.text.isEmpty())
    cp.text = diagram->defaultConductorProperties.text;

This preserves the existing behaviour for labelled conductors (they are cleared to the diagram default so autonumbering can renumber them), while leaving blank conductors alone.

Test plan

• Draw two elements, connect with a conductor, leave the conductor label empty. Copy+paste — pasted conductor should have no label.
• Same setup but give the conductor a label (e.g. "L1"). Copy+paste with erase-label-on-copy enabled — pasted conductor should be reset to the diagram default (usually _).
• Undo/redo the paste — behaviour should be stable.
• Multiple elements pasted at once — conductor reset should happen once, not N times.

Fixes #413.

🤖 Generated with Claude Code

[scorpio810]

Hello @ispyisail have you tryed to remove underscore for default conduction in ,new diagram folio in project setingss, berore? maybe we could delete underscore for conductor 's labels.

[scorpio810]

I ’m fed up with seeing beginners’ projects everywhere with underscores all over the place around the QET conductors...

[ispyisail]

You need to decide how you want it to work, maybe you happy with the exiting code/function? I have provided an alternative which you can reject.